GB1296364A - - Google Patents

Abstract

1296364 Cathode-ray tube circuits RCA CORPORATION 12 Dec 1969 [18 Dec 1968] 60675/69 Heading H4T In a colour television receiver a signal representative of increasing load demands is derived from the (fly-back type) EHT circuit and utilized to vary the gain of the chrominance channel so as to oppose the increasing load demands. As shown, a known colour television receiver is modified by the addition of a resistor 80 connected between the cathode of the colour-burst separator-amplifier 48 and the emitter of transistor 40 (forming an automatic colour control (ACC) circuit 38) the emitter of the latter also being connected to the junction between capacitors 81, 82 which form a voltage divider for the voltage generated across the horizontal efficiency coil 26. During normal operation a keying pulse derived from winding 34 on the horizontal fly-back transformer 20 is effective at the control grid of separator-amplifier 48 to gate the colour-burst from chroma amplifier 50 to the sub-carrier generator 60, 62, 64 the voltage developed at the grid of valve 64 being supplied via resistor 76 to the emitter of ACC transistor 40 which operates when the burst amplitude varies to control the gain of chroma amplifier 50 to maintain the amplitude of the separated burst at a constant value. When, due to conditions caused by changing component values, high temperature &c. the tube 18 attempts to draw an abnormally large beam current the fly-back transformer 20 becomes overloaded and the amplitude of the keying pulses form winding 34 decreases. This results in a decrease in gain of separator-amplifier 48 and the reduced burst amplitude applied to oscillator 64 causes a decrease in the (negative) voltage at the control grid of 64. The emitter of ACC transistor 40 is therefore driven in the positive direction and this results in an increase in the gain of chroma amplifier 50. Additionally, the reduced amplitude of the pulses from the junction of capacitors 40<SP>1</SP>, 42 results in a reduction in the negative voltage developed at the control grid of the valve in the blanker circuit 8 and a further gain increase of chroma amplifier 50 is produced via resistor 80<SP>1</SP>. The effect of both of these gain increases is to make the tube 18 draw even more current from transformer 20 and the resistor 80 and capacitors 81, 82 are scaled to produce a correction or over correction. The operation is as follows. When transformer 20 is overloaded it becomes detuned and the voltage across the efficiency coil 26 increases so that an increase occurs in the voltage applied to the emitter of ACC transistor 40 from the junction of capacitors 81, 82. Also, when the amplitude of the keying pulses from winding 34 is reduced the current through resistor 80 is reduced so that an increase in the emitter circuit occurs and both of these effects result in an increase in emitter current and the voltage at the collector is then effective to reduce the gain of the chroma amplifier 50.